Submillimeter wave heterodyne receiver

ABSTRACT

In an embodiment, a submillimeter wave heterodyne receiver includes a finline ortho-mode transducer comprising thin tapered metallic fins deposited on a thin dielectric substrate to separate a vertically polarized electromagnetic mode from a horizontally polarized electromagnetic mode. Other embodiments are described and claimed.

BENEFIT OF PROVISIONAL APPLICATION

This application is a continuation of and claims the benefit of U.S.patent application Ser. No. 11/820,489, filed 19 Jun. 2007, which claimsthe benefit of U.S. Provisional Application No. 60/814,731, filed 19Jun. 2006.

GOVERNMENT INTEREST

The invention described herein was made in the performance of work undera NASA contract, and is subject to the provisions of Public Law 96-517(35 USC 202) in which the Contractor has elected to retain title.

FIELD

Embodiments relate to radio frequency receivers and waveguidestructures.

BACKGROUND

Dual-polarized, sideband-separating, balanced receivers are well knownand have been used extensively at microwave and millimeter-wavefrequencies. However, it is presently believed that receivers based onprior art designs may not operate well at frequencies at or above 1 THz.Only recently, sideband-separating balanced receivers have been designedfor frequencies beyond W-band (75-110 GHz). For example, receiverarchitectures have been proposed for frequencies up to 900 GHz for theAtacama Large Millimeter/Submillimeter Array (ALMA).

Dual-polarized receivers detect both polarizations of incomingradiation. When both polarizations are received simultaneously, there isa √{square root over (2)} improvement in signal-to-noise ratio (SNR), ora factor of two reduction in observing time. In some prior artapplications, dual polarization operation may be achieved by using awire-grid polarizer to split the telescope beam into two polarizations.The output of a local oscillator (LO) may be injected using abeamsplitter, either after the polarizer, in which case twobeamsplitters are used; or before the polarizer, where a single,correctly oriented beamsplitter is used. Either approach leads to fairlycomplicated optical designs, especially for receivers with multiplebands or multiple pixels.

Most submillimeter-wave receivers in radio astronomy currently usedouble-sideband (DSB) mixers to down convert an RF (Radio Frequency)signal to an intermediate frequency. DSB mixers are useful for continuumobservations, where the signals from both sidebands are equallyimportant. However, for spectral line observations, the presence of thedown-converted signals from the unwanted image band may degrade receiversensitivity and calibration certainty. Therefore, sideband-separatingreceivers with good image rejection capability are desirable for highresolution spectral line observations. Moreover, sideband-separatingreceivers with good image rejection may mitigate confusion from spectrain the image sideband, and may mitigate calibration uncertainty fromsideband imbalance.

Balanced mixers use two or more detector elements in a balancedconfiguration to help suppress local oscillator amplitude modulation(AM) noise, help provide better power handling capabilities thanunbalanced mixers, and help reject certain spurious responses andspurious signals. By simplifying LO injection and eliminating the needfor diplexers, balanced mixers are desirable components for scalingreceivers to multi-pixel arrays.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a receiver architecture with an embodiment ortho-modetransducer.

FIG. 2 illustrates a receiver structure according to an embodiment.

FIGS. 3A and 3B illustrate perspective views of an ortho-mode transduceraccording to an embodiment.

FIG. 4 illustrates a cross sectional view of an ortho-mode transduceraccording to an embodiment.

FIG. 5 illustrates a plan view of an ortho-mode transducer according toan embodiment.

FIG. 6 illustrates a waveguide twist according to an embodiment.

FIGS. 7A-7D illustrate cross sectional views of a waveguide twistaccording to an embodiment.

FIG. 8 illustrates a quadrature hybrid according to an embodiment.

DESCRIPTION OF EMBODIMENTS

In the description that follows, the scope of the term “someembodiments” is not to be so limited as to mean more than oneembodiment, but rather, the scope may include one embodiment, more thanone embodiment, or perhaps all embodiments.

Described are embodiments for a submillimeter-wave heterodyne receiverfor providing dual-polarized, sideband-separating, and balanced outputsignals. In practice, a plurality of embodiments may be configured intoa multi-pixel receiver array. Embodiment receivers may be fabricatedutilizing silicon micromachining, utilizing deep reactive ion etching(DRIE) processes. It is expected that such manufacturing techniques mayyield components working up to 6 THz. For example, embodiments areexpected to provide relatively compact broadband, dual-polarized,sideband-separating, balanced receivers operational at 1.5 THz with morethan a 40% fractional bandwidth.

A receiver according to an embodiment is illustrated in FIG. 1 at a highblock diagram level. An electromagnetic signal is received at hornantenna 102. Coupled to horn 102 is ortho-mode transducer (OMT) 104,followed by six quadrature hybrids: 106, 108, 110, 112, 114, and 116.OMT 104 separates the two orthogonal polarizations of the receivedelectromagnetic signal, with one of the two polarizations (e.g., thevertical polarization) provided to quadrature hybrid 106 and the otherpolarization (e.g., the horizontal polarization) provided to quadraturehybrid 108.

A quadrature hybrid comprises four ports, and may be used in variousways for summing and phase shifting electromagnetic signals. Forexample, two ports may serve as input ports and two ports may serve asoutput ports, where for complex-valued input amplitudes A and B, theoutput signals may be modeled as A/√{square root over (2)}+jB/√{squareroot over (2)} and B/√{square root over (2)}−jA/√{square root over (2)},where j denotes a phase rotation of π/4 (90°). For quadrature hybrid106, one of its ports is terminated by a cooled matched impedance load(labeled “cold load”), so that for an input amplitude A, the output portof hybrid 106 that is coupled to quadrature hybrid 110 providesjA/√{square root over (2)} and the output port of hybrid 106 that iscoupled to quadrature hybrid 112 provides A/√{square root over (2)}. Inpractice, there may be an additional phase shift applied to both outputsignals, but this is not important and is left out for convenience. Thetermination load for hybrid 106 is cooled to reduce thermal noise.Similar remarks apply to quadrature hybrids 108, 114, and 116.

Local oscillator 118 provides a sinusoidal signal to one of the inputports of quadrature hybrid 110. The frequency of local oscillator 118 ischosen so that the frequency difference to ω−f, where ω is the frequencyof the received electromagnetic signal and f is the frequency of thelocal oscillator, is some desired intermediate frequency. Although notexplicitly shown, a bandpass filter, centered about the intermediatefrequency, follows mixer 122, and similarly for mixer 124. The outputsignals from these bandpass filters are summed and provided to an inputport of quadrature hybrid 126. Similarly, local oscillator 118 is alsocoupled to an input port of hybrid 112, whose output ports are coupledto mixers 128 and 130. The outputs of mixers 128 and 130 are bandpassfiltered and summed, and provided to an input port of quadrature hybrid126.

For some applications, the output signals at the output ports ofquadrature hybrid 126 are, expect perhaps for an algebraic sign or anoverall phase shift, the upper and lower sideband components of thereceived electromagnetic signal after shifting down to the intermediatefrequency. Accordingly, the output ports of quadrature hybrid 126 arelabeled in FIG. 1 as “USB V-POL” and “LSB V-POL”, where the “V-POL”denotes vertical polarization, “USB” denotes upper sideband, and “LSB”denotes lower sideband.

For example, for a received signal modeled as Acos (ωt+θ(t)), where ω isthe carrier frequency and the phase modulation θ(t) is a relativelyslowly varying function of time, the output signals at USB and LSBrepresent (up to an overall phase factor or algebraic sign) the uppersideband and lower sideband components, respectively, of Acos (ωt+θ(t))when shifted down to an intermediate frequency. Such a signal model maybe appropriate in CW (Continuous Wave) radar systems, where Acos(ωt+θ(t)) is a received signal from a scatterer.

For many communication systems, the received signal may be expressed asx₁(t)cos(ωt)+x_(Q)(t)cos(ωt), where in general the inphase andquadrature components x₁(t) and x_(Q)(t) are independent of each other,unlike the inphase and quadrature components of Acos (ωt+θ(t)). Then amore general statement is that the output signals at the output ports ofquadrature hybrid 126 are the inphase and quadrature components (up toan overall phase factor or algebraic sign) of the received signal whenshifted down to the intermediate frequency. Stated in another way, theseoutput signals are related to each other by the Hilbert transform (up toan overall phase factor or algebraic sign). The particular timedependence of the inphase and quadrature components x₁(t) and x_(Q)(t)depends upon the chosen modulation scheme for the communication system.

The above discussion for the signal processing of the verticalpolarization also is applicable to the signal processing chain appliedto the horizontal polarization. For some embodiments, local oscillator120 may be the same oscillator as local oscillator 118.

It should be noted that once the signals have been shifted to theintermediate frequency, the functions of quadrature hybrids 126 and 132may be implemented in the digital domain after digital-to-analogconversion. Furthermore, post processing of the output signals fromquadrature hybrids 126 and 132 may be performed in the digital domain.

An embodiment uses a diagonal horn (102), comprising a square waveguide(e.g., 150 μm×150 μm) to accept all polarizations. In one particularembodiment, the horn aperture may be set at 1.27 mm with a 6° semi-flareangle. Horn 102 may be fabricated with DRIE-based siliconmicromachining, where gradient thickness photoresist masking layers maybe used to achieve gradient depths of etching in the longitudinaldirection. The wafer (substrate) in which horn 102 is integrated may bemounted on a 45° tilted stage during the DRIE process to achieve therequired profile in the transverse dimension.

Optical lithography and a DRIE-based silicon micromachining process maybe used to fabricate the horn antenna, OMT, and quadrature hybrids toachieve micron-size waveguide features and sub-micron tolerances, and toachieve planar integration of these components. FIG. 2 illustratesplanar integration on a substrate for some of these components.

FIG. 2 illustrates what may be called split-block integration. Blocks202 and 204 serve as a substrate for the fabrication of various receivercomponents. For some embodiments, blocks 202 and 204 may be silicon inwhich various structures have been etched, and where a good conductor,such as gold, is deposited. In operation, these blocks are joinedtogether at their edges. Some of these structures etched into theseblocks and covered with a deposited conductor may be thought of asrepresenting the “bottom half” of various receiver components, whereother blocks (not shown) are also etched and then deposited with aconductor to form the “top half” of these receiver components. The topand bottom halves are then joined together to complete these receivercomponents. This will be discussed in more detail later. For someembodiments, the fabrication need not be split among two bottom halfblocks, so that a single block (substrate) is etched to form a singlebottom half instead of using two blocks (e.g., blocks 202 and 204) toform the bottom half.

For convenience, we take the convention that the horizontal polarizationis parallel to the plane of blocks 202 and 204, so that the verticalpolarization is perpendicular to this plane. However, this convention isarbitrary, so it is to be understood that the terms horizontal andvertical do not necessarily correspond to the actual physical horizontaland vertical orientations of the final assembled receiver.

For ease of discussion, labels that are used for various receivercomponents in FIG. 1 are also used in FIG. 2 for the correspondingreceiver components. For example, etch structure 102 in FIG. 2represents the bottom half of horn antenna 102 in FIG. 1. Label 104 inFIG. 2 identifies OMT 104 in FIG. 1. More precisely, label 104 in FIG. 2illustrates a portion of OMT 104 that is fabricated on block 202, notthe entire OMT, but for ease of discussion this distinction is not madewhen referring to label 104 in FIG. 2. Similar remarks apply to theother receiver components, keeping in mind that blocks 202 and 204provide only the bottom half of the integration of the receivercomponents. Because of the obvious symmetry, not all components in FIG.2 need be labeled, for the correspondence between the components for thehorizontal polarization processing chain and that of FIG. 1 should beclear.

Cavities 222, 224, 228, and 230 indicate where mixers 122, 124, 128, and130 may be placed. Structure 206 represents the bottom half of awaveguide to guide the sinusoidal output of local oscillator 118 toquadrature hybrids 110, 112, 114, and 116.

FIG. 2 illustrates an enlargement of portions of blocks 202 and 204. Asindicated in the enlargement, OMT 104 is a finline OMT comprising squarewaveguide 208 fitted with thin tapered conductive (e.g., metallic) fins210 and 212. Following OMT 104 in FIG. 2 is transformer 214 comprisingthrough-arm transitions from a square waveguide to a full-heightrectangular waveguide by way of three etched steps. Waveguide 216 guidesthe vertical polarization and waveguide 218 guides the horizontalpolarization, as should be clear by comparing FIG. 2 with thearchitecture of FIG. 1. Following waveguide 216 is waveguide twist 220to rotate the polarization of the transverse electric field vectorspatially by 90° so that the electromagnetic signal provided to the restof the circuit has a polarization parallel to blocks 202 and 204, justas for the electromagnetic signal in waveguide 218. In this way, thesame fabrication techniques may be used for both signal processingchains for the two received polarizations. The structure of OMT 104 andwaveguide twist 220 will now be described in more detail.

FIGS. 3A and 3B illustrate a simplified perspective view of OMT 104,comprising fins 210 and 212. The coordinate system illustrated in FIGS.3A and 3B serves as a reference because later figures will illustratethe receiver components from different perspectives. For ease ofillustration, block 202 is not shown. As will be discussed in moredetail later, fins 210 and 212 are formed on a thin dielectric substrateor membrane, labeled 300 in FIGS. 3A and 3B.

The power flow direction of the input electromagnetic signal isindicated in FIG. 3A by arrow 301, which is taken in the direction ofthe z-axis. This input electromagnetic signal is provided by horn 102,and comprises the two orthogonal polarizations, where the verticalpolarization as referred to in FIG. 1 is along the y-axis and thehorizontal polarization as referred to in FIG. 1 is along the x-axis.The dominant mode at the input port to the OMT for both of thesepolarizations is the TE₁₀ mode.

The dashed lines in FIG. 3A indicate the edge of waveguide 208underneath fins 210 and 212. Note that portions of fins 210 and 212extend outside of the dashed lines. These portions may be referred to asbeamleads. The beamleads are positioned adjacent to (that is, on top of)block 202 during assembly. These beamleads are bonded to block 202 wherewaveguide 208 is etched into block 202, so that fins 210 and 212 areheld in place. This bonding may be van der Waal bonding. There is a gap(302) between fins 210 and 212 adjacent to dielectric substrate 300 topropagate a finline mode.

When the top half block is bonded to block 202 to complete the OMTstructure with waveguide 208, fins 210 and 212 are positioned in a planeparallel to the x-y plane and gap 302 essentially runs along the centerof waveguide 208, follows bend 303, and then out through side-arm 305(see FIG. 3B). The straight portion of the OMT may be referred to as thethrough-arm. FIG. 3B provides a wire-grid view of OMT 104 insidewaveguide 208, indicating input port 307, output pot 311 for theside-arm, and output port 309 for the through-arm.

The horizontally polarized TE₁₀ mode provided at input port 307 to theOMT is parallel to fins 210 and 212 (in the x-axis direction), andgradually transforms to a finline mode as it propagates along the OMT.Its energy is essentially confined to the narrow gap (302) between fins210 and 212 in the center of waveguide 208. This energy then may beremoved from waveguide 208 by curving the finline (narrow gap 302) andbringing it out through a side wall of waveguide 208. This isillustrated by bend 303 and that portion of fins 210 and 212 near labels304 and 306. The narrow gap starts to widen when the finline mode isguided out of waveguide 208 and through side-arm 305. The finline modegradually transforms into a TE₁₀ mode, where it is guided to the rest ofthe signal processing chain for the horizontally polarized portion ofthe received electromagnetic signal.

The vertically polarized TE₁₀ mode provided input port 307 to the OMT isorthogonal to fins 210 and 212 (in the y-axis direction), and passesthrough the OMT essentially unperturbed when the fins are sufficientlythin. Resistive card 308 is used to suppress the excitation of unwantedmodes at the termination of the fins in the through-arm of the OMT, andmay be fabricated by depositing a resistive film on the same dielectricsubstrate that the fins are deposited on. For the vertically polarizedmode, the through-arm of the OMT transitions from a square cross sectioninput port 307 to a full-height rectangular waveguide at output port 309by way of three-step matching transformer 214.

For some embodiments, for the horizontally polarized TE₁₀ mode, thefull-height rectangular side-arm (where fin portions 306 and 304 exit)uses a mitered 45° bend so that both waveguide structures at outputports 309 and 311 may be in the same plane. For some embodiments, a 40%height waveguide iris may be used at the junction between the side-armand through-arm to minimize the effect of the side-arm opening on thevertical polarization signal, while mitigating disturbance of thefinline guide mode for the horizontal polarization.

As can be seen from the previous illustrations, the overall design ofthe OMT is planar, and by using DRIE-based silicon micromachiningtechniques, it is expected that operation at THz frequencies isfeasible. This structure, however, suffers from somewhat higher lossthan the so-called Beifot type designs due to Ohmic losses in the fins.Fortunately, the resistive losses in a normal metal fin may be reducedif quantum-limited mixers are operated at cryogenic temperatures.

Some embodiments may use metallic fins photolithographically etched on athin silicon dielectric substrate. This is illustration in FIG. 4, whichprovides a simplified cross sectional view of OMT 104. The orientationof the coordinate axes is illustrated in FIG. 4, where the z-axis pointsinto the page of the illustration, so that the illustration in FIG. 4 isa slice of the illustration in FIG. 3B in a plane perpendicular to thez-axis close to the input port of the OMT. The polarizations of the twoelectric fields are shown, where electric field E₁ has a horizontalpolarization (in the x-axis direction) and electric field E₂ has avertical polarization (in the y-axis direction). Top half block 402 isbonded to bottom half block 202 by van der Waal forces to complete theOMT and waveguide structure.

FIG. 4 shows a gap between half blocks 402 and 202 due to the thicknessof fins 210 and 212. However, in practice, for some embodiments fins 210and 212 are about 1 micron thick, so that when half blocks 402 and 202are put together, they squeeze down fins 210 and 212 without leaving agap.

Fins 210 and 212 may be fabricated by depositing metal on dielectricsubstrate 300, followed by etching to produce the desired shape of thefins. For some embodiments, the dielectric substrate may besilicon-on-insulator (SOI), or for example GaAs (Gallium Arsenide), andmay have a thickness from 2 to 3 μm. The choice for the width of thefinline gap (302) depends upon the operating frequency. For example, forsome embodiments, the gap may be 25 μm for a carrier frequency of 100GHz, and may scale accordingly, where for example the gap may be 2.5 μmfor 1 THz.

A single gold metallization layer may be deposited on the SOI substrateand etched to form the fins. The beamleads may be formed from backsideprocessing by etching away the SOI substrate using a separate mask. Forexample, free-standing metallic beamleads have been discussed in G.Chattopadhyay, et al., “An All-Solid-State Broad-Band FrequencyMultiplier Chain at 1500 GHz,” IEEE Trans. Microwave Theory Tech., vol.52, no. 5, pp. 1538-1547, May 2004. These beamleads are relativelymechanically strong, reliable, and are expected to provide very goodgrounding when mounted between two mating split-blocks. The beamleadsare also expected to help in handling the finline circuit duringassembly, which may have a thin layer of gold on a 1 μm thick SOIsubstrate.

For some embodiments, the main through-arm of the OMT at the input portis a square waveguide of dimension 150 μm by 150 μm, with a taperedwaveguide-to-finline transition, and the finline mode is taken through a45° bend and out through a reduced height guide (the side-arm). It wasfound that a 45° finline bend minimized mode conversion, and improvedcross-polarization performance.

Embodiment OMTs are expected to offer good performance from 1200 to 1800GHz. The input match is approximately −20 dB across the band for bothpolarizations. The insertion loss is approximately 1.5 dB for thevertical polarization and approximately 0.6 dB for the horizontalpolarization. Simulated isolation and cross-polarization levels for theOMT were found to be in the −50 dB range.

FIG. 5 illustrates yet another view of OMT 104. FIG. 5 is a plan view ofOMT 104, with fins 210 and 212 bonded on top of block 202. The dashedlines indicate the edges of waveguide 208. From the coordinate axesshown in FIG. 5 it is seen that the y-axis points out of the page of theillustration, and the polarizations of the electric fields areindicated. For simplicity, substrate 300 is not illustrated. FIG. 5illustrates some of the points in the above discussion, showing theside-arm at about a 45° bend from the through-arm of the OMT to separateout E₁ from E₂, where E₂ propagates out along the through-arm.

Because the electromagnetic signals for the through-arm and side-armhave orthogonal polarizations, waveguide twist 220 may be used to bringthe electromagnetic signal from the through-arm of the OMT (thevertically polarized received signal) to the same plane as theelectromagnetic signal from the side-arm of the OMT (the horizontallypolarized received signal), so that integration with the quadraturehybrids is easily facilitated. Waveguide twist 220 has a structure withstepped transitions to rotate the waveguide orientation for thevertically polarized received electromagnetic signal so that the signalbecomes polarized in the horizontal direction.

FIG. 6 illustrates the bottom half and top half structures for waveguidetwist 220 according to an embodiment, where a one step vertical (y-axisdirection) transition is etched in the bottom half, and a one stepvertical transition is etched in the top half. (The bottom and tophalves are indicated in the illustration.) The input ports are denotedby an arrow denoting the power direction, along the z-axis. Theorientation of the coordinate system in FIG. 6 is not necessarily thesame as the coordinate system in the previous illustrations. Theperspectives are not meant to be exact, but to illustrate the steptransitions. The hatched area in the bottom half is meant to convey avisible vertical wall of the bottom etched structure. With the top halfbonded to the bottom half, there are two steps in the verticaldirection. Because there only two steps in the vertical direction with acurved waveguide profile in the horizontal (x-y) plane, it may befabricated using the DRIE technique.

Lines 602 and 604 are meant to convey the edges of the verticaltransitions for the bottom and top halves, respectively. A verticaltransition etch has some relatively constant height in the y-axisdirection, but where the width in the x-axis direction graduallyincreases from zero at the input port, to the full width at the outputport. One way to view the top half is to rotate the bottom half aboutthe z-axis by 180°. FIGS. 7A through 7B illustrate cross-sectional viewsof the waveguide twist illustrated in FIG. 6, where the coordinatesystem in FIGS. 7A-B has the same orientation as the coordinate systemin FIG. 6. The views in FIGS. 7A-B are slices of the waveguide twisttaken perpendicular to the z-axis. FIG. 7A illustrates the input port,and FIG. 7D illustrates the output port. FIGS. 7B and 7C illustrate twoslices taken somewhere between the input and output ports.

FIG. 6 illustrates a waveguide twist comprising only one verticaltransition for each half block. However, some embodiments may employmore than one vertical transition for each half block. For the top halfblock of such embodiments, the length of a vertical transition is lessthan the length of the vertical transition just below it. A similarremark applies to the bottom half block for such an embodiment.

As discussed previously, embodiments use waveguide quadrature hybridsfor the balanced sideband-separating mixer designs. This architectureuses a total of three RF quadrature hybrids for each polarization; onefor the sideband separation and one for each of the two balanced mixers.A waveguide form for a quadrature hybrid comprises two parallelwaveguides coupled through a series of apertures or branch waveguides.Some embodiments may use this branch line coupler design because of itsbroad bandwidth, low loss, and compatibility with DRIE-micro-machinedsplit blocks.

The amplitude and phase imbalance at the outputs of a quadrature hybridmay affect the local oscillator noise injection of the balanced mixerand the image rejection of the sideband separating mixer. A design goalshould be to develop waveguide quadrature hybrids with less than 1.5 dBof amplitude imbalance and less than a few degrees of phase imbalanceover a wide frequency band at terahertz frequencies.

Important design parameters for the hybrid are the width of the branchguides, the spacing of the branches, and the branch separation distance.For some embodiments, the design has branch guide widths in the 10 to 20μm range, the spacing of the branches in the 30 to 60 μm range, and thebranch separation distance approximately 55 μm. FIG. 8 provides awire-grid illustration of an embodiment waveguide quadrature hybrid.

As discussed previously, various receiver components may be fabricatedusing DRIE techniques. DRIE of silicon has become an importanttechnology process in micro-fabricating components which range in depthfrom 10 μm to greater than 1 mm. There are various well-known methodsone may employ for DRIE. A popular method is commonly known as the“Bosch” process. See, for example, U.S. Pat. No. 5,501,893, “Method ofAnisotropically Etching Silicon, by F. Laermer and A. Schilp. This is adry process, compared to other deep etching techniques which rely onanisotropic wet chemistry. The Bosch process uses the deep reactive ionetching technique and does not appear to be sensitive tocrystallographic orientation, and this process may have a very highselectivity to the masking material. This process is also believed to beconsiderably safer than the equivalent wet process.

The DRIE process uses a fluorine based gas chemistry (sulfurhexafluoride, SF₆) to etch the silicon, combined with a fluorocarbonprocess (octofluorocyclobutane-C₄F₈) to provide sidewall passivation andimproved selectivity to the masking material. A complete etch processcycles between etch and deposition steps many times to achieve deep,vertical etch profiles.

Some embodiments were developed using the following DRIE fabricationmethod for developing silicon micro-machined components. A highresistivity silicon wafer is coated with a 5 μm thick layer of SJR-5740positive photo resist (a product of MicroChem Corp.), and exposed forapproximately 40 seconds using a 25 W-cm⁻² 320 nm ultraviolet (UV)light. The waveguide etching mask is aligned with a Karl-Suss MA6aligner, followed by developing in a mixture of AZ400K (a product of AZelectronic materials) and de-ionized water in a 1:3 ratio until thepattern clears. The wafer is then mounted on a backing wafer using acrystal bond. The etching rate is approximately 2 μm/min, and achieves auniformity of about ±10% across the wafer. The second step is similar tothe first, with the exception that extra precaution should be used whilecoating resist onto the backside. This is because the high resistivitysilicon wafer is brittle, and the first DRIE step makes it thinner. Astandard blue masking sheet may be used to cover the etched side, andmay be removed immediately after coating the resist. The second etch isa through wafer etching step, and the final parts are released inacetone.

Fabrication tolerances for horn 102 and OMT 104 assembly are notexpected to be necessarily stringent because there are no tuningstructures for these devices. Alignment crosses should be etched intothe silicon structure to facilitate assembly. For some embodiments,after a DRIE etch, the silicon waveguide split-block halves (blocks 202and 204) are gold plated with an e-beam evaporator, after masking offthe alignment marks. Bonding is not necessary to hold the finline chipwith its beamleads. Alignment of the top split-block with the bottomhalf may be achieved with an infrared semiconductor alignment tool, alsoknown as flip-chip bonder. This tool holds both the top and bottomhalves of the chip in air chucks on precision motion stages. An infraredmicroscope looks through the transparent silicon at the location of thealignment crosses to allow registration of the top and bottom halves ofthe structures. The air chucks then clamp the halves together with theVan der Waals forces bonding the gold metallization layers of thesplit-blocks. The silicon blocks may be glued and clamped into a copperfixture for testing.

Various modifications may be made to the described embodiments withoutdeparting from the scope of the invention as claimed below.

1. An apparatus comprising: a dielectric substrate having a first edgeand a second edge; a first planar conductive fin deposited on thesubstrate, and having a portion extending over the first edge of thesubstrate; a second planar conductive fin deposited on the substrate,and having a portion extending over the second edge of the substrate, sothat a gap is formed between the first and second conductive fins andadjacent to the dielectric substrate; and a first substrate comprising aconductive surface having a first trench, the first trench having athrough-arm having a first end and a second end, and the first trenchhaving a side-arm extending away from the through-arm at a side positionbetween the first and second ends of the through-arm, the side-armhaving an end; wherein the portions of the first and second planarconductive fins extending over the first and second edges of thedielectric substrate are adjacent to the first substrate such that thedielectric substrate and the gap are positioned over the first trench.2. The apparatus as set forth in claim 1, the gap transitioning from afirst wide width over the first end of the through-arm to a narrow widthover the side position, and transitioning to a second wide widthpositioned over the end of the side-arm.
 3. The apparatus as set forthin claim 2, further comprising: a second substrate comprising aconductive surface having a second trench, the second trench having athrough-arm having a first end and a second end, and the second trenchhaving a side-arm extending away from the through-arm of the secondtrench at a side position between the first and second ends of thethrough arm, the side-arm of the second trench having an end; whereinthe second substrate is adjacent to the first substrate so that thefirst and second trenches form a waveguide, wherein the first ends ofthe straight-through arms of the first and second trenches form an inputport to the waveguide, the second ends of the straight-through arms ofthe first and second trenches form a first output port of the waveguide,and the ends of the side-arms of the first and second trenches form asecond output port to the waveguide.
 4. The apparatus as set forth inclaim 3, the first and second substrates each comprising silicon, wheretheir conductive surfaces are formed by depositing metallization on thesilicon.
 5. The apparatus as set forth in claim 3, the dielectricsubstrate comprising a material selected from the group consisting ofGallium Arsenide and silicon-on-insulator.
 6. The apparatus as set forthin claim 3, further comprising: a resistive material deposited on thedielectric substrate adjacent to the first and second planar conductivefins, positioned near the second ends of the straight-arms of the firstand second trenches.
 7. The apparatus as set forth in claim 6, whereinthe input port has a square profile.
 8. The apparatus as set forth inclaim 7, wherein for a TE₁₀ mode propagating into the input port with atransverse electric field vector polarized in a direction parallel tothe planar conductive fins, the TE₁₀ transitions into a finline modenear the narrow width of the gap, and transitions into a TE₁₀ mode atthe second output port with a transverse electric filed vector polarizedin a direction parallel to the planer conductive fins.
 9. The apparatusas set forth in claim 8, wherein a TE₁₀ mode propagating into the inputport with a transverse electric field vector polarized in a directionperpendicular to the planar conductive fins exits the first output portessentially unperturbed.
 10. The apparatus as set forth in claim 1, thefirst substrate having a third trench having a first end and a secondend, the second substrate having a fourth trench having a first end anda second end, so that the third trench and the fourth trench form awaveguide having an input port and an output port, the input port of thewaveguide coupled to the first output port, where the first ends of thethird and fourth trenches form the input port of the waveguide having arectangular shape with an input width and an input height, and thesecond ends of the third and fourth trenches form the output port of thewaveguide having a rectangular shape with an output width and an outputheight, where the output height is greater than the input height, thewaveguide having a first vertical step with a width that transitionsfrom zero at the input port of the waveguide to the output width at theoutput port of the waveguide, and the waveguide having a second verticalstep with a width that transitions from zero at the input port of thewaveguide to the output width at the output port of the waveguide. 11.An apparatus comprising: an ortho-mode transducer comprising: first andsecond substrates each having a conductive surface such that whencoupled together form a waveguide comprising an input port; a firstoutput port; and a second output port; a dielectric substrate; and apair of planar fins deposited on the dielectric substrate and bonded tothe first and second substrates, the pair of planar fins forming a gapsuch that a TE₁₀ mode with an electric field vector polarized parallelto the planar fins and propagating into the input port transitions to afinline mode and then propagates out of the second output port, and aTE₁₀ mode with an electric field vector polarized perpendicular to theplanar fins and propagating into the input port propagates out of thefirst output port.
 12. The apparatus as set forth in claim 11, the firstand second substrates each comprising silicon having depositedmetallization to form the conductive surfaces.
 13. The apparatus as setforth in claim 11, the dielectric substrate comprising a materialselected from the group consisting of Gallium Arsenide andsilicon-on-insulator.
 14. The apparatus as set forth in claim 11,further comprising: a waveguide having an input port coupled to thefirst output port of the ortho-mode transducer, and having an outputport, the input port having a rectangular shape with an input width andan input height, and the output port having a rectangular shape with anoutput width and an output height, where the output height is greaterthan the input height, the waveguide having a vertical step with a widththat transitions from zero at the input port to the output width at theoutput port.
 15. An apparatus comprising: a first substrate having aconductive surface, the first substrate having a first trench having afirst end and a second end; and a second substrate having a conductivesurface, the second substrate having a second trench having a first endand a second end, so that the first trench and the second trench form awaveguide having an input port and an output port, where the first endsof the first and second trenches form the input port of the waveguidehaving a rectangular shape with an input width and an input height, andthe second ends of the first and second trenches form the output port ofthe waveguide having a rectangular shape with an output width and anoutput height, where the output height is greater than the input height,the waveguide having a first vertical step with a width that transitionsfrom zero at the input port of the waveguide to the output width at theoutput port of the waveguide, and the waveguide having a second verticalstep with a width that transitions from zero at the input port of thewaveguide to the output width at the output port of the waveguide.